The goals of Project 4 are to assess the therapeutic potential of novel 0X1 receptor antagonists with the most desirable pharmacological and physiochemical properties. In Specific Aim I we will test the effects of novel 0X1 receptor antagonists on intravenous (IV) nicotine self-administration behavior in rats. We will will test their effects on cue-induced reinstatement of extinguished nicotine-seeking behavior, and in attenuating the incubation of craving for nicotine. In complementary experiments we will also examine their effects on food-motivated behaviors using almost identical procedures. In this manner we can identify novel 0X1 receptor antagonists that selectively decrease nicotine addiction-related behaviors in rats, and also those that act in a non-specific manner through generalized disruption of behavioral performance. Nicotine stimulates brain reward circuitries, reflected in lowering of intracranial self-stimulation (ICSS) thresholds in rats, and obtaining this action of nicotine may motivate the initiation and persistence of the tobacco habit. Under Specific Aim 11, we will test the effects of novel 0X1 receptor antagonists on nicotine-induced lowering of ICSS thresholds in rats. Using the ICSS procedure we will also be able to identify 0X1 receptor antagonists with intrinsic abuse potential as those compounds that induce nicotine-like lowering ICSS thresholds. Finally, a major concern when developing novel antagonists for 0X1 receptors as potential therapeutics, or for any other potential drug target in brain, is that they may have unknown off-target actions that contribute to their behavioral effects. As such, positive data generated in animal mode s of nicotine addiction using such compounds would be entirely confounded and have little clinical utility. In order to test for the behavioral selectivity of novel 0X1 receptor antagonists we will test their effects on intravenous nicotine self-administration behavior in wildtype mice and in mice with null mutation in the 0X1 receptor (i.e., knockout mice). It is predicted that selective 0X1 receptor antagonists will decrease nicotine intake in wildtype mice without altering intake in knockout mice.